1. Field of the Invention
The present invention relates to an information recording medium that allows at least one of recording and reproduction of information by optical or electrical means and a method for producing the same.
2. Description of the Related Art
As a conventional information recording medium, the inventors of the present invention have developed and commercialized 4.7 GB/DVD-RAM (Digital Versatile Disk-Random Access Memory), which is a large capacity rewritable phase change information recording medium that can be used as a data file and an image file (e.g., JP2001-322357). FIG. 6 shows the structure of this information recording medium (DVD-RAM). The information recording medium shown in FIG. 6 has a multilayered film having a seven layer structure in which a first dielectric layer 102, a first interface layer 103, a recording layer 4, a second interface layer 105, a second dielectric layer 106, a light-absorption correction layer 7 and a reflective layer 8 are formed on one surface of a substrate 1 in this order. In this information recording medium, the first dielectric layer 102 is provided in a position nearer to the laser light incident side than the second dielectric layer 106. The first interface layer 103 and the second interface layer 105 have the same positional relationship. Thus, in this specification, when an information recording medium includes at least two layers having the same function, the layers are referred to as “the first”, “the second”, “the third” . . . sequentially, starting from the layer that is at the nearest side when viewed from the incident laser light.
The first dielectric layer 102 and the second dielectric layer 106 serve to adjust the optical distance to increase the optical absorption efficiency of the recording layer 4 and increase the difference in the reflectance between when the recording layer 4 is in a crystalline phase and when the recording layer 4 is in an amorphous phase so as to increase the signal amplitude. A mixture of 80 mol % of ZnS and 20 mol % of SiO2 (hereinafter, expressed by “(ZnS)80 (SiO2)20 (mol %)” in this specification; other mixtures also are expressed in the same manner) that conventionally has been used as a material for a dielectric layer is amorphous, has a low thermal conductivity, is transparent and has a high refractive index. (ZnS)80 (SiO2)20 (mol %) also has a high film-formation speed when a film is being formed, and good mechanical characteristics and moisture resistance. Thus, (ZnS)80 (SiO2)20 (mol %) is an excellent material as a dielectric layer of an information recording medium.
When the thermal conductivities of the first dielectric layer 102 and the second dielectric layer 106 are low, the heat generated when laser light is incident on the recording layer 4 is hardly diffused in the in-plane direction of the dielectric layers 102 and 106. In particular, when the thermal conductivity of the second dielectric layer 106 is low, the heat is diffused in the thickness direction from the recording layer 4 to the reflective layer 8 rapidly, so that the recording layer 4 is cooled in a shorter time, which makes it easy to form an amorphous mark (recording mark). When the recording mark is hard to form, it is necessary to record information with a high peak power, whereas when the recording mark is formed easily, information can be recorded with a low peak power. Thus, when the thermal conductivities of the first dielectric layer 102 and the second dielectric layer 106 are low, information can be recorded with a low peak power, and thus the recording sensitivity of the information recording medium can be increased.
When the thermal conductivities of the first dielectric layer 102 and the second dielectric layer 106 are high, information is recorded with a high peak power, and thus the recording sensitivity of the information recording medium becomes low. The dielectric layers 102 and 106 are present in the form of a thin film having such a small thickness that its thermal conductivity cannot be measured accurately. Therefore, the inventors of the present invention use the recording sensitivity of the information recording medium as a relative determination basis that provides the magnitude of the thermal conductivity of the dielectric layers.
The recording layer 4 is formed of, for example, a material that is crystallized in a high speed including Ge—Sn—Sb—Te. An information recording medium having such a material as the recording layer 4 has not only excellent initial recording performance, but also excellent archival characteristic and archival overwrite characteristic. Information is recorded, erased and rewritten in a rewritable phase change information recording medium by utilizing the fact that a reversible phase change is caused between a crystalline phase and an amorphous phase in the recording layer 4. When the recording layer 4 is irradiated with laser light with a high power (peak power) and cooled, the irradiated portion is converted to an amorphous phase and a recording mark is formed. When the recording layer 4 is irradiated with laser light with a low power (bias power) to be heated and then cooled gradually, the irradiated portion is converted to a crystalline phase and the recorded information is erased. The already recorded information can be erased and rewritten to new information by irradiating the recording layer with laser light whose power is modulated between the peak power level and the bias power level. The repeated rewriting performance is indicated by the maximum number of times of repetitions of rewriting within a range that causes no problem of jitter values in practical use. When this value is larger, the repeated rewriting performance is better. In particular, it is desirable for information recording media for data files to have good repeated rewriting performance.
The first interface layer 103 and the second interface layer 105 serve to prevent substance movement that might be caused between the first dielectric layer 102 and the recording layer 4 and between the second dielectric layer 106 and the recording layer 4 (see JP10-275360A and International Publication No. 97/34298 regarding layers that serve to prevent substance movement). The first and the second interface layers 103 and 105 prevent S atoms of (ZnS)80(SiO2)20 (mol %) contained in the first and the second dielectric layers 102 and 106 from being diffused to the recording layer 4 while information is being rewritten by irradiating the recording layer 4 with laser light. It is known that when a large amount of S atoms are diffused to the recording layer 4, the reflectance of the recording layer 4 is reduced, and the repeated rewriting performance deteriorates (e.g., see N. Yamada et al., Japanese Journal of Applied Physics Vol. 37(1998) pp. 2104–2110).
The light-absorption correction layer 7 serves to adjust the ratio Ac/Aa of the light absorptance Ac when the recording layer 4 is in a crystalline state and the light absorptance Aa when the recording layer 4 is in an amorphous state, and prevent the mark shape from being distorted at the time of rewriting. The reflective layer 8 serves to facilitate change of the recording layer 4 into amorphous, optically by increasing the light amount that is absorbed by the recording layer 4 and thermally by diffusing heat generated in the recording layer 4, rapidly to cool the recording layer 4 rapidly. Furthermore, the reflective layer 8 also serves to protect the multilayered film from the use environment.
Thus, the information recording medium shown in FIG. 6 has a structure in which the seven layers functioning in the above-described manner are laminated so that excellent repeated rewriting performance and high reliability can be ensured at a large capacity of 4.7 GB, and thus has been commercialized.
As described above, when (ZnS)80(SiO2)20 (mol %) is used for the first and the second dielectric layers and the first and the second dielectric layers are formed so as to be in contact with the recording layer 4, S atoms of (ZnS)80(SiO2)20 (mol %) contained in the first and the second dielectric layers are diffused to the recording layer with laser light while information is being rewritten by irradiating the recording layer. Thus, the reflectance of the recording layer is reduced, and the repeated rewriting performance deteriorates. Therefore, it is necessary to provide an interface layer for suppressing substance movement between the recording layer and the dielectric layers in order to ensure the repeated rewriting characteristics as in the information recording medium shown in FIG. 6. However, in view of the price of media, the smaller the number of the layers constituting a medium is, the more desirable it is. This is because a reduction in the number of layers can achieve a reduction in the material cost, compactness of production apparatuses, and an increase in production volume by reducing the production time, which consequently leads to a reduction in the price of the medium.
The inventors of the present invention examined the possibility of eliminating at least one interface layer of the first interface layer and the second interface layer as a method for reducing the number of layers. The interface layer is a very thin layer having a thickness of, for example, 2 nm to 5 nm, and is structurally very fragile. Therefore, film rupture is caused while information is being recorded repeatedly, and consequently atomic diffusion tends to occur. Therefore, it also is desirable to eliminate the interface layers in terms of the stability of the information recording medium. In order to reduce the number of the interface layers to a smaller number, it is necessary to form the dielectric layers with a material that does not allow movement of S, that is, a material system that is free from S atoms.
Regarding the material of the dielectric layers, the following is desirable: (1) having some transparency with respect to light of a wavelength for recording and reproduction in order to ensure sufficient absorption of light to the recording layer for efficient recording and sufficient reflected light for reproducing recorded information satisfactorily; (2) providing recording sensitivity equal to or more than that of an information recording medium containing a multilayered film having a seven layer structure in which an interface layer is provided; (3) being thermally stable and having a high melting point so as not to be melted by repeated rewriting; (4) having a high film-formation speed at the time of forming a film in order to ensure productivity; and (5) having excellent reliability.
The present invention is directed to providing an information recording medium having good repeated rewriting performance by providing a material layer in which substance movement to the recording layer is suppressed and that has good adhesion with the recording layer, even if it is formed in contact with the recording layer without providing an interface layer.